The present invention relates to a technique for testing or inspecting a property or aspect of a sample such as a wafer. In more detail, the present invention relates to an electron beam apparatus applicable to a defect detection and/or line width measurement of a wafer during a semiconductor manufacturing process and so on, in which
electron beams are irradiated to a sample, secondary electrons emitted from the sample and varying according to a property of the sample surface are captured, and image data is created therefrom to evaluate patterns on the sample surface with a high throughput on the basis of theimage data. The present invention also relates to an evaluation system and a semiconductor device manufacturing method, both of which utilize the electron beam apparatus. In the present description, the meaning of the term “evaluation” of a sample also includes the meaning of“inspection” such as defect detection and line width measurement of a sample.
In semiconductor processes, design rules are now going to enter the era of 100 nm, and the production scheme is shifting from small-kind mass production represented by DRAM to a multi-kind-small production such as SOC (silicon on chip). Associated with this shifting, the number of manufacturing steps has been increased, and an improved yield of each process is essential, so that testing for defects caused by the process becomes important.
With the trend of increasingly higher integration of semiconductor devices and finer patterns, a need exists for high resolution, high throughput testing apparatuses. A resolution of 100 nm or less is required for examining defects on a wafer of 100 nm design rule. Also, as manufacturing steps are increased in response to the requirement of higher integration of devices, the amount of testing is increased and thus a higher throughput is required. Further, as devices are formed of an increased number of layers, testing apparatuses are required to have the ability to detect defective contacts (electric defect) of vias which connect lines on layers to each other. While optical defect testing apparatuses are mainly used at present, it is anticipated that electron beam based defect testing apparatuses will substitute for optical defect testing apparatus as a dominant testing apparatus in the future from a viewpoint of the resolution and defective contact testing capabilities. However, the electron beam based defect testing apparatus also has a disadvantage in that it is inferior to the optical one in the throughput. For this reason, a need exists for the development of a high resolution, high throughput electron beam based testing apparatus which is capable of electrically detecting defects.
It is said that the resolution of an optical defect testing apparatus is limited to one half of the wavelength of used light, and the limit is approximately 0.2 μm in an example of practically used optical defect detecting apparatus which uses visible light. On the other hand, in electron beam based systems, scanning electron microscopes (SEM) have been commercially available. The scanning electron microscope has a resolution of 0.1 μm and takes a testing time of eight hours per 20 cm wafer. The electron beam based system also has a significant feature that it is capable of testing electric defects (broken lines, defective conduction of lines, defective conduction of vias, and so on). However, it takes so long testing time that it is expected to develop a defect testing apparatus which can rapidly conduct a test. Further, a testing apparatus is expensive and low in throughput as compared with other process apparatuses, so that it is presently used after critical steps, such as after etching, deposition (including copper coating), CMP (chemical-mechanical polishing) planarization processing, and so on.
A testing apparatus in accordance with an electron beam based scanning (SEM) scheme will be described. An SEM based testing apparatus narrows down an electron beam which is linearly irradiated to a sample for scanning. The diameter of the electron beam corresponds to the resolution. On the other hand, by moving a stage in a direction perpendicular to a direction in which the electron beam is scanned, a region under observation is tow-dimensionally irradiated with the electron beam. In general, the width over which the electron beam is scanned, extends over several hundred μm. Secondary electron beams emitted from the sample by the irradiation of the focussed electron beam (called the “primary electron beam”) are detected by a combination of a scintillator and a photomultiplier (photomultiplier tube) or a semiconductor based detector (using PIN diodes). The coordinates of irradiated positions and the amount of the secondary electron beams (signal strength) are combined to generate an image which is stored in a storage device or output on a CRT (Braun tube). The foregoing is the principle of SEM (scanning electron microscope). From an image generated by this system, defects on a semiconductor (generally, Si) wafer is detected in the middle of a manufacturing procedure. A detecting speed corresponding to the throughput, is determined by the intensity of a primary electron beam (current value), a size of a pixel, and a response speed of a detector. Currently available maximum values are 0.1 μm for the beam diameter (which may be regarded as the same as the resolution), 100 nA for the current value of the primary electron beam, and 100 MHz for the response speed of the detector, in which case it is said that a testing speed is approximately eight hours per wafer of 20 cm diameter. Therefore, there exists a problem that a testing speed is significantly low in comparison with that in an optical based testing apparatus. For instance, the former testing speed is 1/20 or less of the latter testing speed.
If a beam current is increased in order to achieve a high throughput, a satisfactory SEM image cannot be obtained in the case of a wafer having an insulating membrane on its surface because charging occurs.
As another method for improving an inspection speed, in terms of which an SEM system is poor, there have been proposed SEM systems (multi-beam SEM systems) and apparatuses employing a plurality of electron beams. According to the systems and apparatuses, an inspection speed is improved in proportion to the number of electron beams. However, as a plurality of primary electron beams impinge obliquely on a wafer and a plurality of secondary electron beams are pulled from the wafer obliquely, only secondary electrons released obliquely from the wafer are caught by a detector. Further, a shadow occasionally appears on an image and secondary electrons from a plurality of electron beams are difficult to separate from one another, which disadvantageously results in a mix of the secondary electrons.
Still further, there has been no suggestion or consideration about an interaction between an electron beam apparatus and other sub-systems in an evaluation system employing a multi-beam based electron beam apparatus and thus, at present there aren't any complete evaluation systems of a high throughput. In the meantime, as a wafer to be inspected becomes greater, sub-systems must be re-designed to accommodate to a greater wafer, a solution for which has not yet been suggested either.